Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 268

3 years (2007-9) of combined carbon dioxide enrichment and warming (comparing
present ambient controls with elevated temperatures). So, did the C 3 plants thrive

more than the C 4 ? It turns out that they did not.
What Morgan et al. (2011) found was that, compared with C 3 grasses, C 4 grasses
growing under semi-arid climatic conditions will prosper in a world with higher
carbon dioxide concentrations and (the 'and' is crucial) warmer temperatures. This
is because of the difference in the way C 3 and C 4 plants can manage water. The leaf
pores, stomata, of C 4 plants open less widely under high carbon dioxide semi-arid
warm conditions than those of C 3 plants, so C 4 plants tend to transpire less water
vapour. (See also a review of this work by Dennis Baldocchi, 2011.) The conclusions
have implications for ecosystem function and ecosystem service (such as grazing
provision).
So far, in our all-too-brief survey of current global warming's biological dimen-
sions, we have looked at terrestrial dimensions. But what is happening to global
productivity in the oceans? Aside from being of ecological interest, because oceans
contribute roughly half of global primary productivity, this is of practical relevance
due to our reliance on the ecosystem services the oceans provide. Not least of these
are mundane economics including fishery potentials and the fundamental importance
of planetary oxygen regulation.
Although there is a volume of research literature on phytoplankton, the key ocean
primary producers, only recently have we been able to gain a truly global appraisal
of the situation through space remote sensing. (Sampling using point source, a slow-
moving ship [or even a small fleet], cannot provide a global snapshot.) In 2006 US
researchers Michael Behrenfeld, Robert O'Malley, David Siegel et al. used NASA's
Sea Viewing Wide Field-of-view Sensor, known as SeaWiFS, the first ocean colour
sensor with a wide view, launched in 1997. Its spectral analysis capabilities allow it to
assess the amount of chlorophyll in the sea surface, hence it can be used to estimate
stocks and productivity. Primary productivity is small compared to total phytoplank-
ton stock so detecting annual changes requires accurate long-term measurement.
Behrenfeld and colleagues did just this over the period from 1997 to the beginning
of 2006. From the annual temperature plot of global temperature in Figure 5.4 you
may be able to see the (ENSO-enhanced) rise in global temperature between 1996-8
of a little over 0.4 ◦ C followed by more stability to 2006 near the high end of this
temperature hike. Comparing this with both total global marine primary productiv-
ity and its distribution, the Behrenfeld team were able to gather some preliminary
results (a longer-term study would be welcome) and from these they drew some ini-
tial conclusions. They showed that global marine net primary productivity (roughly
55 GtC year − 1 ) grew with temperature by some 1.93 GtC year − 1 . They then continued
to track it, as it declined a little in the years to 2005. Between 1999 and 2004 they
not only found a smaller decline in primary productivity of 0.19 GtC year − 1 , but also
that there was a difference between high latitudes (very roughly above 40 ◦ where
more of ocean primary productivity takes place) and the tropical and subtropical low
latitudes. Specifically, the low latitudes are where the bands of temperature-stratified
ocean occur, where the warm surface lies over the cool, abyssal depths. At the high
latitudes the ocean is more unstratified, because there is more oceanic vertical mixing
due to up- and down-welling. Seventy-four per cent of the Earth's stratified oceans